Nanostructured alloys, those having grain size smaller than 10−7 meter, often exhibit improved hardness, strength, ductility, diffusivity, and soft magnetic properties in comparison to traditional heat precipitation and dispersion strengthened alloys.
As with traditional alloys, nanostructured alloys undergo the processes of recovery, recrystallization, and grain growth upon heating. Recovery is the relief of a portion of the stored internal energy of a material after it has been plastically deformed through dislocation motion. Recrystallization is the formation of new, strain-free, equiaxed grains from previous strain hardened grains, driven by stored internal energy of the strained grains. Grain growth reduces the overall stored energy of the alloy by reducing the number of high-energy grain boundaries.
Nanostructured alloys are most often prepared by high-energy ball milling. In room temperature ball milling, the localized high temperatures encountered during collision of the balls causes recovery within the alloy, which counters the effect of further deformation. To prevent such recovery, nanostructured alloys are processed under cryogenic conditions, i.e. cryomilling, such as in a bath of liquid nitrogen, which effectively cold-works the particles. The cold-working introduces numerous dislocations, which form subgrain boundaries, and eventually high-angle grain boundaries with grain sizes on the order of nanometers.
During cryomilling, the grain size of the metal does not decrease indefinitely. Eventually, the grain size of the metal reaches an equilibrium state after which no amount of cold working will decrease the grain size of the metal below the equilibrium grain size. Equilibrium grain diameters as small as approximately 2.5×10−8 meter have been observed via electron microscopy and measured by x-ray diffraction at this stage in processing. After cryomilling, the metal powders are nanostructured alloys that have high-ductility and a low recrystallization temperature.
To create a useful metallic article out of the cryomilled powder, the powder is consolidated and thermo-mechanically processed into a solid, dimensionally desirable form. An exemplary thermo-mechanical process is hot isostatic pressing (HIPping), and other thermo-mechanical techniques are known in the art of metal working.
During HIPping, and any subsequent extrusion and/or forging of the metal, recovery, recrystallization, and grain growth each occur within the metal article. These changes have, heretofore, been considered an unavoidable consequence of the thermo-mechanical processing that may negatively effect the qualities of the resulting article.
It is desired to provide a method of producing a high strength metal alloy having improved qualities over and above those metal alloys created from traditional cryomilled metal powders. It is further desired to provide a method of producing a metal alloy having improved qualities over those metal alloys created by using traditional thermo-mechanical processes to treat traditional cryomilled alloys.